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How does a flight or squadron communicate when radio silence is in effect?
By MSgt. David N. Craft [Public domain], via Wikimedia Commons
How does a flight or squadron communicate when radio silence is in effect?
If one pilot spots an unexpected target or danger, how is it communicated to their flight?
Rocking the wings? Kicking the rudder? A gun burst? What are the different non-verbal ways of communicating in such scenario?

Welcome to the site @DominicC. I'm not sure what you are asking, what scenario do you have in mind?

USAF Instruction 11-205 Aircraft Cockpit and Formation Flying Flight Signals details the guidance and procedures for Air Force formation flight. It also indicates distress signals that have been standardized for use with North Atlantic Treaty Organization (NATO) forces.
To get attentions, usually the wings are rocked:

Attention in the air Execute rapid shallow rocking of wings.
Loosen Formation Fishtail the aircraft.

The following are some of the distress reasons and corresponding signals:

Bailing Out or Ejection One or both clenched fists pulled downward
  across the face to simulate pulling an ejection face curtain.
Intercepting Signals The  intercepting  aircraft  positions  itself  in
  front of and usually to the left of the intercepted aircraft and rocks its wings.  This is a signal that the interceptor wishes the other aircraft to follow it.  A responding irregular flashing of all available lights in this case indicates distress

In case of unexpected danger/target or where time is critical, the flight is allowed to break radio silence. The instructions states:

These signals reduce radio communications and permit pilots to communicate with each other during periods of radio silence.  Use the radios if the situation permits, when time is critical, or the visual signals are creating confusion. (emphasis mine)

There were certain hand signals that I do remember from my years flying off the USS Nimitz in the 1980's. The ones I remembered were the most commonly used, e.g. what is your fuel status, reducing power, ascend, descend, gear down (for section approaches), gear up (for section takeoffs), etc. We flew with one another much like a family, and like a family we also had hand signals internal to the squadron that we used.
I wanted to reference a fairly complete guide of signals and found this beautiful training manual online: The Formation Pilot's Knowledge Guide. If you enjoy aviation it is worth downloading and at least skimming through. Very thorough treatment of formation flying. Really well written and diagrammed. See Appendix I for the hand signals, and find these 3 pages attached at the end of this text for convenient reference.
Before going on to some comments about in-flight communication let me note at least one difference in hand signals from what is referenced and what I used in the squadron. We used a different set of hand signals for answering a fuel status check. We gave our fuel state in pounds instead of time. The numbers 1-5 were signed with vertical fingers. The numbers 6-9 were signed with horizontal fingers, and a zero was a clenched fist. So there are certainly differences to be expected, but the list gives an idea of what situations are generally accounted for in standardized hand signals.
It is important to understand that when you are in formation as the wingman your responsibility is to stay on the lead's wing and maintain physical separation from their aircraft. Additionally, with a good prebrief and a steady lead the flight could take place without much communication. For example, during take off you knew your responsibilities, and you knew what things were to happen and when. So, for example, if the lead missed the hand signal for gear down on a section landing, it usually wasn't a big deal, although albeit a bit of a surprise. Or the wingman might be involved in their own tasks within the cockpit and miss the leads signal. That happened to me at least once, but hey, his gear were coming down. Mine came down in time without disrupting the flight. The same was true in the rendezvous circle, or in an attack. You stayed on the leads wing and protected the flight! We called it flight integrity.
As Naval aviators we took pride in terse radio communications. There were limited departure and approach frequencies, as well as ground frequencies. Each launch included between 15 to 20 aircraft, as I remember, and they all shared the same frequencies. There were times when we launched the Air Wing for a practice War-at-Sea Strike. In this case there might be as many 30 or more aircraft airborne. We kept radio communications to the minimum, often using mic clicks to communicate. The only time you might here "chatter" was while tuned the squadron's radio frequency, or on some pre-briefed frequency that the flight might use, but this wasn't common. We were pretty busy fulfilling our mission, and there wasn't a lot of time to chat.
A bit off-topic, but still dealing with communications without radios was EMCON procedures. There were times when the carrier exercised what is called Emissions Control, or EMCON. In this scenario aircraft took off, performed their missions, and landed without communications with the ship. The ship listened, and that is all it did. All electromagnetic emissions were shut down. It became a passive communicator, and capturing signals. And so here we are not talking about plane-to-plane communications, but silent aircraft to ship procedures (controller).
The 2 most difficult transitions were coming back to the ship and the pattern. It is a big ocean, and when I was flying the radar we had did not do a good job at discriminating targets on the sea. You had to find the ship, and it was fairly easy to lose the ship. There were tricks we used. Also, the landing pattern is a very crowded place with flights stacked on top of one another over the ship. The pilots controlled the pattern. One A7 squadron was the first down, and this was because in case the deck wasn't ready they usually had enough fuel to go around without getting into serious trouble. The F14's were above us, since they typically had lower fuel reserves, A6's were towards the top. The very crowded pattern was controlled by the pilots, all with little fuel reserves. This is easy enough during the day, but at night it can get pretty intense.
For completeness I will mention that there are a whole litany of hand signals used on the flight deck to manage the flow of traffic. This includes start up of the aircraft, taxiing, and then catapult procedures, as well as procedures for coming out of the wire. These can be found online fairly easily. The only time there was any communication on the deck was when there was an emergency. As a pilot you might be up a secondary frequency to get something straight with the flight members, but that was rare. You listened to your departure channel. The only call I remember making was when I got airborne.
Towards the terminal phase of the launch procedure you were taxied up and parked behind the aircraft getting ready to launch. That way it took only a minute or 2 after they took their shot to get you in the catapult and off the deck. One of the last things the aircraft in front of you does before taking the cat shot is to go to military power. To keep the aircraft waiting from getting damaged from the powerful jet exhaust big deflectors rise up out of the deck and direct it  upwards. My mate was behind the deflector while an A6 was on the catapult at full power. All of a sudden the JBD lowered and the powerful exhaust caught his aircraft and pushed it towards the side of the deck. Okay, he used the radio. I can still hear him calling for "AJ401 CHOCKS, CHOCKS!"
Finally, it is worth mentioning this following example demonstrating how hand signals are developed out of a need, and in the end not always institutionalized. When we ask the question what hand signals there are and how they are used we can identify a set of institutionally accepted signals, as detailed below. These signals are derived from a need of pilots to communicate aircraft-to-aircraft. After a time these signals became codified. I had the honor of listening to several POW's describe their experiences in Vietnam. My CAG Commander had been a prisoner in Vietnam and I had the opportunity to talk with him about his experience in a personal setting. One thing I remember is that there was a POW who knew sign language and the prisoners used his knowledge within the specific need to build their own set of signals that could be used to communicate across the distances of the compound.

You're welcome. But accidentally you rejected it and I did it again ;-) Great answer btw and I like the drawings.

Yes. Sorry, and I am happy that you caught my comment :)


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Pilot announcement PA quality
On a recent passenger flight my son asked me, "How come we're able to build a 737 for \$50M - \$80M with fantastic reliability and technology but can't design a PA system so that we can understand the pilot's announcements?" Many are quite un-intelligible but the cabin crew announcements generally are intelligible. I didn't have a good answer. Does anyone know?
As a follow on and of more concern, if pilots' microphone technique / equipment is so poor then how does air-traffic control understand them? I know there is a limited vocabulary set and that standard phrases are used to avoid mis-hearing but there must be some problems.

Is this an actual question? I've had no problem hearing the announcements so far. It seems the microphone is good enough to work for ATC, as there is only one. You may listen to ATC feeds to convince yourself (and your son).

@mins as there is only one no there isn't, and I suspect that this might be the problem.  All of the big iron cockpits I am familiar with have a hand mike, and I've seen the flight crew use this for cabin address in preference to the headset boom mike.  I suspect that the maintenance on this might not as rigorous as for the headsets and, if the quality is poor, does anyone bother to raise a maintenance snag for it?

@Simon: Right. Also there is a pair for each pilot. But my point was they use the same device for radio and PA. I understand from your comment this is not the case in general.

@mins.  they use the same device for radio and PA.  Sorry to be a pain, no they don't.  There is almost no commonality other than the ability to select either mike as the input source to PA or radio.  The comms selector panel is the only common piece and this is little more than a set of switches.

My guess would be the pilot was using a handheld mic and was holding it way too close to his mouth. You'd be surprised that a lot of people don't know how to hold a microphone correctly.

@Simon: As I wrote (maybe not clearly): I understand from your comment this is not the case in general

Related: http://aviation.stackexchange.com/questions/15222/why-is-the-sound-quality-of-onboard-public-address-so-bad

Much discussion about the cockpit equipment, but what about speakers in the cabin, their placement and such? Aircraft and other cabin, passenger noise.

@kevin: I am a live sound engineer somewhere in the middle between advanced amateur and semi-pro, and I can attest to that. "Normal" speakers pretty much do not understand how to hold a microphone. Even singers often have gross misunderstandings about the physics of a directional microphone. Rappers are the worst, by far. Bottom line: pretty much nobody knows how to hold a microphone.

The duplicate answers only the first question, but not at all "if pilots' microphone technique / equipment is so poor then how does air-traffic control understand them?". Voting to reopen.

@JörgWMittag Boy I second that! I used to do live sound too. The worst is when people can't seem to figure out that putting their hand over the mic is causing the feedback.

@mins It seems the microphone is good enough to work for ATC When I listen to ATC I'm utterly amazed the controller can understand half of what the pilots are saying. I've chalked it up to their experience in deciphering it because ATC coms are anything but clear.

@TomMcW: Select a Tower frequency to prevent distance-related loss/noise to occur. Listening at KSEA Tower is good for me, albeit English is not my mother tongue. By good I mean good for 3kHz bandwidth AM.

@mins I listen to MEM tower all the time. The radio interference is less, but I still have a hard time understanding the pilots. Heck, when I used to jumpseat there was a headphone that I could plug in directly into the com. I still couldn't understand them! Of course, back then I had no idea of the terminology so they could have been speaking French for all it meant to me. It helps to know what they might be saying.

Many pilots prefer to use their headset boom mic or hand mic instead of the dedicated interphone handset for PA announcements.
I don't know why, but I know from personal experience that the dedicated interphone handset has much better PA readability.

That doesn't really explain why (supposedly) the announcements are worse than pilots messages with ATC. I doubt the microphone bandwidth is different (the radio channel yes, but not the microphone).

Sorry, I don't have any knowledge as to why the ATC microphone does not work as we'll as the dedicated interphone handset.

Anyway, this question has already been answered here: http://aviation.stackexchange.com/questions/15222/why-is-the-sound-quality-of-onboard-public-address-so-bad

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What is the tailhook on naval aircraft made out of?
What is the tailhook that enable naval aircraft to land on aircraft carriers, it must be made out of a strong metal to not bend or 'snap' under the force of the thrust that the plane is expelling, as they land then put thrust to full.

Image Source—PACIFIC OCEAN (Dec. 22, 2011) Landing signal officers supervise the arrested recovery of an F/A-18C Hornet on the flight deck aboard the Nimitz-class aircraft carrier USS Carl Vinson (CVN 70).

Looking at the specs, it is steel. Looking at the price, it must be unobtainium.

You may also refer the Wikimedia image, there is a description with it.

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Does the size of control surfaces affect maneuverability, and if they are smaller, is it easier to stall?
Does the size of control surfaces effect maneuverability and make it easier to stall?

As the lift force produced by a control surface is proportional to its surface, the manoeuvrability is affected by their size.

There are at least 3 possible cases and unfortunately your question doesn't tell which one you would consider valid.
The maneuverability (the maximum rate of rotation?) can be measured by the moment of the force exerted by the surface at the CG.
Let's consider the area of the control surface is smaller, and...

All other parameters are unchanged, in particular surface airfoil, center of pressure, and angle of attack:

As the currently selected answer tells, the maximum lift available from the control surface is lower, hence the maximum moment is lower, and the maneuverability is reduced.

As the airfoil and the angle of attack are unchanged, the stall characteristics are unaffected.

This theoretical case is unlikely to be seen as the designer would certainly compensate the reduction of the area by some other change.

The surface is moved to a location further from the CG to maintain the moment with the smaller lift force:

Neither maneuverability nor stall characteristics are affected.

The angle of attack is increased to generate more lift and compensate for the smaller area:

Maneuverability is decreased as the maximum lift obtained with the maximum angle of attack is decreased.
Stall is easier to trigger as the angle of attack (for an equal moment) is closer to the stall angle.

any of the four cases!

If by 3 you mean increased deflection, then the airplane stall will be (more or less) unaffected. (I suspect the OP meant airplane stall). It may be easier to stall the control surface itself, but not in all cases (e.g. when pulling up near the airplane stall). And if we don't approach surface stall, then the overall control efficiency (and thus manoeuvrability in this sense) can be pretty much equal.

@Zeus: Thanks for the comment (I believe that's correct for elevators, but is less applicable to canards). I agree my answer is not a great one and needs to be refined.


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Are there any 'smart' ejection seats?
Are there any 'smart' ejection seats that eject automatically when an engine failure occurs or the aircraft gets into to a stall that can't be gotten out of, to save the pilot the decision of whether s/he should try and save the aircraft?

That is the last thing I would want as a pilot. Engine failures are not necessarily "eject" events. Stalls are relatively easy to get out of. There are a lot of situations where timing with regards to external environment are just as important as timing with respect to the aircraft.

@ymb1: Sounds like a duplicate.

The Soviet Yakovlev Yak-38 (and also the Yakovlev Yak-141) had an automatic ejection seat. The aircraft had two smaller engines for VTOL. If one of them failed, the seat ejected automatically once the aircraft rolled past 60$^\circ$. From airvectors.net:

The pilot sat on a K-36VM ejection seat, with the ejection performed automatically in takeoff or landing excursions using an "SK-EM" system. The SK-EM system was automatically engaged after the aircraft rose a few meters from the deck; it could be turned off manually, or would turn itself off if the vectored-thrust nozzles were more than 67 degrees to the vertical.

These ejection seats were first developed for Yakovlev Yak-36 (though never installed on them). The Yak- 36 had two side-by-engines, the failure of any one of which could flip the aircraft- this led to the development of an automatic ejection system that would activate aircraft seemed to be departing from safe vertical-flight parameters.
According to ejection-history.org.uk, 18 automatic ejections have been recorded in Yak-38s.


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Why do some helicopters, e.g CH-53, have some kind of hook thing protruding from the underside of the tail section, what is this for?
Why do some helicopters have some type of hook sticking out of the bottom of the tail section, my guess is that it is used to secure the helicopter to carrier deck, but i'm not sure, so I am going to ask in case I'm wrong.

Source: www.b-domke.de

Prevents a tail rotor strike in a tail-low landing.

Related: 14 CFR 29.411 (a) It must be impossible for the tail rotor to contact the landing surface during a normal landing.

The device is often referred to as a tail rotor guard or tail skid, which hints at its purpose.
Helicopters that have tail rotors that sweep below the tail boom are usually equipped with this guard to prevent the tail/rotor from striking the ground during landing, both during a rolling landing or a vertical landing. It is especially useful for landing off-field where the terrain may not be flat near the tail rotor.
Many rotorcraft, both general aviation, transport, and military have this feature, and it may even be required by regulation (see 14 CFR 29.411 for transport category rotorcraft).
Here is the tail rotor guard on a Robinson R44:

Source: Airport-Data.com
While technically the same as in the original question, I do not believe the R44 rotor sweeps below the vertical fin. The Red/White guard protects the blade while the "stick" on the fin keeps the fin from contacting the ground. It may be possible to contact the rotor if landing on soft earth or tall grass...
You can search through Google Images to find many examples of rotor guards.


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What is the largest aircraft ever put in a spin?
What is the largest aircraft, by weight, ever deliberately put into a spin?

Mil Mi-26 or Zeppelin I believe. It's relatively easy and kind of inevitable during normal operation.

I suppose the op is asking about fixed-wing...

@Sanchises I thought the previous comment was a joke.  But in case it is not, the definition of spin includes the stall of a wing, so it must have a wing.

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What does a ski jump takeoff feel like?
Some aircraft takeoff from aircraft carriers using a ski jump.  What does it feel like to takeoff this way?  Does it feel like an alpine ski jump?  How does the feeling compare to catapult takeoff?

"feel" could be objectively measured with an accelarometer.

If I provided you with a plot of acceleration against time would it tell you what it feels like? If I told you it's like an alpine ski jump, but you've never done an alpine ski jump, would that tell you how it feels? I don't think this question is answerable in its current form.

Asking for a qualitative description is not the same as asking for an opinion. VTLO.

You will have a hard time finding anyone who has done both.

@CatchAsCatchCan "f I provided you with a plot of acceleration against time would it tell you what it feels like?" Me, yes.  Other people, maybe not.  "but you've never done an alpine ski jump" Well, I have, so this does not apply.

@AnonymousPhysicist while we try to answer the question for the benefit of the asker, the aim of any SE site is to provide knowledge to as many people as possible in a format accessible to everyone.

@Federico I did not wish to dictate the format of the answer.  If someone posts an overly technical answer, I can help make it more accessible.  The point I was making is that "what does takeoff feel like" is not opinion-based.

Go grab a skateboard, head to the nearest skate park and ride it at high speed off one of the ramps.

It's exactly like an alpine ski jump except instead of coming down and landing right away, you keep going. Hopefully. :)

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How are winter weather operations different if they are based on an aircraft carrier instead of a runway on land?
I am guessing that naval aircraft carriers do not have room for conventional deicing trucks and snowplows.  If they need to have frequent takeoffs and landings, time for clearing precipitation may be very limited, and there may be no alternate runways.  How do they deal with winter weather?
I am interested in treatment of both the carrier deck and the aircraft.

It's true, carriers typically don't have snow removal and deicing equipment (like trucks, but they do have fluid and spray equipment). They don't have this because they regularly avoid weather that could cause significant icing or snow (carriers move pretty quick and weather is somewhat predictable).
But it certainly isn't unheard of that they encounter snow or ice.
The carrier deck
The deck is somewhat cleared using tools like shovels:

Source: TheDrive.com
They also have a "sweeper" truck that can be used to sweep smaller amounts of snow off the deck:

Source: TheDrive.com
This is an older picture, but the equipment is still on modern carriers. They use it primarily for cleaning the deck and after incidents requiring broad FOD removal.
So one thing is in abundant supply on carriers, manual labor, and a lot of snow is removed doing just that. Teams working with shovels and dumping it overboard.
The Russians have a tool primarily used for FOD clearing, but is pretty effective at deicing the carrier deck:

This is basically a jet engine (same one used in MIG-15's) that is mounted to the front of a tractor with a diffuser on the front.
Aircraft
The aircraft have an advantage that most of them can be stored inside when they have icing conditions. Aircraft that are caught outside are manually cleaned using things like brooms and even baseball bats.

Source: TheDrive.com
They also have manual de-icing spray systems:

Source: US National Archives

"one thing is in abundant supply on carriers, manual labor" The Army calls that "Private power". My eldest participated in that program when he was at Ft. Drum. I'd presume that clearing the flight deck is more for the safety of the manpower than for the aircraft on a modern carrier. Take off roll is dictated by the catapult, and landing distance by the catch wire. It's only once the plane needs to be taxied that surface conditions really matter.


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How do airliner pilots practice recovering from a bounced landing?
Other questions say bouncing an airliner is bad, why bounces happen, and how to recover from a bounce.  How do pilots practice bounce recovery?

I am guessing that the answer is "in a simulator."  If that's the answer, how is the simulator configured to cause the bounce?  And who does the configuring?

Airline pilots practice everything in a simulator, both in initial training as well as every year or so for recurrent training.
A friend of mine is a new sim instructor (and retired captain) at an airline, and he learned to teach this exact lesson yesterday. To ensure the plane bounces, they configure extreme wind shear at a certain altitude above the runway, so the plane essentially stalls and slams into the pavement.
He tried it first at 30ft, and the pilot easily handled the small bounce—too easily, he thought. He then tried 50ft, and the impact was so severe that the sim went “off motion” to protect itself from damage, and the (simulated) plane did not survive. So, the correct setting seems to be somewhere between those two values.

I am curious what "extreme" means.

The wind speed used probably depends on the make of the plane and the speed at that moment, to get 'perfect' stall-like conditions and force a bounce.

@AnonymousPhysicist True anecdote: we were investigating cracks developing in military jets used for training navy pilots operating from  aircraft carriers. We had no real idea what g forces were involved, so we instrumented a plane and got a flight instructor to perform a series of deliberate "heavy landings."  The guy chickened out at about 10g, so we set the measurement system to record up to 30g. Eight of the first ten landings flown by the trainees were "off the scale" !!!!

There is the "bounced landing" button available for the instructor in the IOS of the flight simulator

I am referring to Level D flight simulators

Can you provide some evidence? What's the IOS?


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How are floatplanes deiced?
If a floatplane, seaplane, or flying boat has ice on the wings, how is the ice removed before takeoff?  Can it be done while keeping the plane in the water?

Poring hot water on the wings was a very common de-ice practice in the bush. It’s probably still used.

When I was a bush pilot, this would be a problem late in the season, and I'd come to the dock and find the plane covered in heavy frost. The temperatures were just below freezing and starting to rise above freezing, so post-freezing wasn't a problem, and I'd just climb on top with a bucket on a rope, and haul up lake water (which is still fairly warm in late October) and throw it across the wings.
If you had ice on the plane from freezing rain overnight, you'd need to use a lot more water, but if it was still freezing rain, or if freezing rain was in the forecast, you weren't going anywhere in any case.
Sometimes late in the year, the freezing level might be at 1000 ft, and you could go flying in rain, but you had to stay low, below the freezing level, even if the cloud base was well above. You could cruise at say, 500 ft (a typical altitude in bad weather all year), see the OAT showing just above 0 degrees, climb a few hundred feet, and as soon as the OAT crept below 0 ice would start to accumulate on the windshield and back down you'd go.
With trips in the morning, you can count on the freezing level moving up so it's safe to go in marginal conditions, but in the afternoon, if the conditions were borderline, you knew it could only get worse and at some point you have to cancel your trips.
Bush flying is truly the most live-by-your-wits-in-a-world-of-shades-of-grey type of flying.


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Is inverted aerial refeuling possible?
Google found lots of information about how to air-to-air refuel an inverted plane inside a video game.  Is it possible in reality?

This could be potentially a good idea for some airplanes. Some jet fighters were known to be very difficult to plug in the refueling pipe and it may take many test flights to figure out the best way of doing it, some of which turned out to be real bizarre. If flying inverted is the solution then so be it.

Any of the large tanker aircraft like a KC-135, KC-10, KC-46, KC-130, etc can't sustain inverted flight, so those would be out.
The most likely case would be a tactical aircraft refueling another with a "buddy pod" setup. The US Navy uses S-3 aircraft, and before they retired, A-6 aircraft, for this mission. If the plumbing was set up in the tanker aircraft to pick up fuel from the top of the tank, then this could probably work. Whether any of those aircraft are actually plumbed to do that, would be a question for an S-3, A-6, or now-a-days an F/A-18 driver.
Since there's no conceivable utility to accomplishing such a procedure, I rather doubt any designers put extra effort or expense into enabling it.
All that said, nothing much would prevent the tanker & receiver from continuing the refueling during a barrel roll, which maintains positive G throughout, so for a moment, you'd be refueling while inverted, just without negative G's.  Again, no value in doing that, but to be able to say "they were refueling while inverted" then that's probably the easiest way to do it.
EDIT:
If only the receiver is inverted, the issues would be different. Refueling with a boom & receptacle wouldn't work, since the receptacle is on the top of the receiver. With a probe & drogue, the pilot of the receiver would probably have almost none of the tanker aircraft in view - rather than it being visible "above" him through his canopy, it's now obscured by the nose and the floor of his own aircraft. Flying formation on only a drogue sounds hard; flying formation on just a hose (ie after the drogue is attached to the probe) sounds impossible.

The question is for an inverted aircraft receiving fuel.  I was thinking the tanker would probably be upright.  Unusual tanker attitudes are interesting, though.

The point about the boom makes sense.  I do not find your explanation of probe and drogue convincing.  For a KC-130, the drogue is off center.  An inverted receiver might be able to view the KC-130 by looking to the side, where these is no nose.  The F-16 cockpit has a pretty good field of view, and nose cameras (EFVS) are a thing that exists.  Maybe the F-35 AN/AAQ-37 DAS would be sufficient for formation flying?

https://media.cnn.com/api/v1/images/stellar/prod/200706105211-voyager-refueling.jpg?q=w_1015,c_fill/f_webp


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What weather conditions have been tested with a wind tunnel?
If I want to know how my aircraft component performs in different atmospheric conditions, I might want to test it in a wind tunnel.  What sorts of weather conditions have been tested in a tunnel?
I am thinking about 1 m scale wind tunnels or bigger, not tiny ones.
I saw Boeing offers an icing tunnel.  I found some very old specifications (p. 6).  It does not say if the humidity can be controlled.  The temperature range -25° to 50°F does not cover the heat aircraft often encounter.  Are there other capabilities?

NASA Glenn has tunnels with a very wide range of performance.  Here are some impressive specs to take a look at:  https://www1.grc.nasa.gov/facilities/irt/   https://www1.grc.nasa.gov/facilities/10x10/

Note: do not overestimate wind tunnels. Computer simulation, and then real tests are the most important parts. Wind tunnels cannot provide accurate results if you do not use a full scale model (and you can build a model and attach it to a real aircraft to validate all weather conditions).

Yes, icing tunnels can run at very cold temperatures and can introduce water vapor in quantities sufficient to represent humidity, clouds, and precipitation.
Wind tunnels often control pressure and temperature of the flow.  The first pressure tunnels provided the first chance to understand the effect of Reynolds number on airfoils.
Wind tunnels are usually cooled (rather than heated) -- the temperature in a closed circuit wind tunnel will rise to very hot temperatures if it is not cooled (or shut down to cool).  Closed circuit tunnels also often exchange a fraction of the air on each circuit to reduce the cooling requirements.
Some wind tunnels run gas other than air.  Sometimes this is as simple as Nitrogen - which is basically air, but high speed tunnels sometimes run Helium, Argon, CO2, and other gasses.
Instead of running at high pressure, wind tunnels for testing engines often run at very low pressure (and temperatures).  This way, they can simulate conditions for the engine operating at high altitude.
These tunnels aren't necessarily trying to model 'weather', but they are trying to represent the ambient conditions important to the system being studied.
For example, a spacecraft re-entering the edge of the atmosphere experiences an ambient condition of rarefied gas (where the air is so thin, the continuum assumption falls apart) at about Mach 26.  By the time the spacecraft reaches the earth's surface, it will have experienced everything down to incompressible Mach zero flow at sea level.
At times during that re-entry, the flow will be so hot, the gas will convert to a charged plasma that interferes with radio communication.
No single wind tunnel can duplicate all of these conditions, but there are wind tunnels and other related ground test equipment to simulate each of these phenomena that are used in the development of spacecraft and aircraft.

"Wind tunnels are usually cooled (rather than heated) -- the temperature in a closed circuit wind tunnel will rise to very hot temperatures if it is not cooled" Where did you get this information?

Think about the control volume of a moderate closed circuit wind tunnel.  Suppose it has a 100 Hp motor driving the fan.  Since the flow can not accelerate to infinite speed (due to friction), that is 100 Hp of work that eventually turns into heat.  You're putting lots of work in, but aren't taking any heat out.  I have been in several wind tunnels with cooling apparatus.  https://www.grc.nasa.gov/www/k-12/airplane/tuncret.html  "Hotter running conditions than an open return tunnel. Tunnel may have to employ heat exchangers or active cooling."

Good information.  The difference between active and passive cooling is huge - which is it in practice?

There are closed circuit tunnels with at least four (and combinations of the four) approaches -- do nothing, simply limit run times.  Exchange some fraction of air with cool ambient air (not a fully closed return).  Use various approaches to passive cooling.  Use elaborate heat exchangers with cooling towers to actively cool.  Most large wind tunnels are one-off designs tailored to the targeted flow conditions, the site, and the cost of materials and labor at the time of construction.  In practice, it is an OR -- not a single answer.


text:
In what ways does the current FAA written exam differ from the Sporty's practice test?
I am preparing to take my PPL written exam. I have been using the Sporty's practice exam program to gauge my progress? Recently I heard that the FAA has changed a few things about the tests. So I am curios: How the current written exam differs from the Sporty's practice exam?

Does the current written exam use the Airman Knowledge Testing Supplement for Sport Pilot, Recreational Pilot and Private Pilot?
Are the questions formatted in the same way as the practice question?
If you perform well with the Sporty's practice exam with an actual understanding of how you got to your answer, would you likely do well with the current test?

I took the PPL written exam this morning (9/30/2016), but I used the Jeppesen PPL materials to prepare. Although I cannot speak to the Sporty's study materials, the Jeppesen practice exam and supporting materials were identical to the questions on the real exam. I would assume that Sporty's will be (almost?) the same. I actually scored higher on the real exam than on any of the practice tests.
Yes, the current exam uses the Airman Knowledge Testing Supplement for Sport Pilot, Recreational Pilot and Private Pilot booklet.


text:
How does an aircraft distribute fuel equally between powering the turbines on the engine and injecting it into the afterburner?
How does an aircraft distribute its fuel equally between powering the engine turbines and injecting it into the afterburner?

The aircraft fuel system does not  distribute fuel equally among the afterburner and the engine's core (i.e. the compressor-combustion chamber-turbine part). It distributes fuel in such a way that the combustion is maintained in both the combustion chamber and afterburner and the required thrust is produced.
Consider the case of F-15 E/I/S or F-16 C/D powered by PW F100-PW-229 (data from jet-engine.net). Here, the thrust and SFC (specific fuel consumption) for dry and wet thrust are given as 17800/29100 lb and 0.726/2.060 lb/lb/hr respectively.
Simple calculation yields fuel consumption rate of 59946 lb/hr and 12923 lb/hr with and without afterburners. This answer by @KeithS gives the fuel consumption of F-16 with and without afterburner as 64000 and 8000 lb/hr respectively. Also note that fuel consumption varies greatly with other conditions like altitude, external stores etc.
It can easily be seen that the use of afterburner increases fuel consumption by 5-8 times and most of this has to go into the afterburner. So, once the pilot punches the afterburner, the fuel system delivers much more fuel to the afterburners than to the 'core' of the engine. These decisions are made usually by the aircraft computer/engine FADEC based on pilot input in all modern combat aircraft.

Why would fuel consumption vary depending on external stores?

@fooot Extra drag. Assuming we're talking about fuel consumption to maintain a given airspeed with other factors equal, the extra drag produced by the external stores would require more power (and thus more fuel) to overcome.


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Could a UAV ever be hacked and piloted?
I am currently reading a book at the moment, and I got to a point were a UAV was hacked, piloted, released its pay load and crashed, could this ever happen in real life?

I don't think there's any good answer to this: almost anything is possible in theory, but in reality a lot depends on the specific details. Which UAV? Who are the attackers? What resources do they have? Etc. We do have a few questions on hacking already, and you might get a better response on security.SE about general wireless security and remote hacking

Just assume that if one person (or group) is smart enough to design the security protocols, somewhere, somewhen, there will be another person (or group) smart enough to defeat the security protocols. Applies to all technology from your ATM PIN to FaceTwit to drones.

Not only is it possible to hack a UAV, but it has already happened. In December 2011 a US RQ-170 recon drone landed in Iran after being hijacked by Iran's security services, possibly with Russian help. This was as much poor security implementation on the drone as clever hacking on the part of Iran.
As for whether it would be possible to take over an armed drone and fire its weapons, it would certainly seem so.

Of course, everything with an external connection is vulnerable. That is the reason why always pilots should control a plane with humans. GdD already posted a well known example.
You shouldn't connect any important control-systems to an external network, this is especially important for planes, cars, nuclear power plants and so on.
As an example a really autonomous car which should improve security, should be independent from the internet and a satellite signal. For example a hacker doesn't need to hack your car, is enough to manipulate the external maps (e.g. osm, google or here) to kill a lot of people within seconds.

Logging data in a read only mode is not dangerous and exchanging data need not be dangerous, if the control-system doesn't trust this data.


text:
How does the NATO code name system work, are they just random names, or is there some sort of rule(s)?
When an aircraft is designated a NATO code name, are there a certain rule(s) that contribute to the naming of an aircraft.

From Wiki

They provide unambiguous and easily understood English words in a
  uniform manner in place of the original designations, which either may
  have been unknown to the Western world at the time or easily confused
  codes.

Much the like the NATO alphabet rules the names but be easy to understand (possibly under multiple speaking accents) and must be non ambiguous.
The following rules are also applied

Aircraft
The first letter indicates the type of aircraft, like
  Bear for a bomber aircraft, or Fulcrum for a fighter aircraft.
F —
  fighter aircraft, also later ground attack aircraft: List of NATO
  reporting names for fighter aircraft
B — bomber aircraft: List of NATO
  reporting names for bomber aircraft
C — commercial aircraft and
  airliners, and cargo aircraft: List of NATO reporting names for
  transport aircraft
H — helicopters: List of NATO reporting names for
  helicopters
M — Miscellaneous; trainers, reconnaissance, seaplanes,
  tankers, AEW etc.: List of NATO reporting names for miscellaneous
  aircraft
For fixed-wing aircraft, one syllable names were used for
  propeller-powered craft (turboprops included), while two-syllable
  names indicated jet engines.


text:
What is this intake at the back of a 737?
I've circled an intake in red on the images below. I am working on a 737 NG and I don't know what it's for. I am thinking it's a pressure probe for APU control but I am not sure.

Source

Front view: comparing location to aft door confirms it's the same "device." (Full image.)

When you say "working on" a 737NG, are you actually performing  maintenance on it?  If so, have you opened the maintenance manual for that aircraft?  Further that point, why are you not asking the senior 737 A&P  mechanic at your company?  If you are building a model, this question makes more sense.

Please provide in image source

No, it is not a probe, it is called APU AIR INLET SCOOP and it's purpose is to cool/vent the area between the APU insulation blanket and the bulkhead. See AMM 49-10 (SDS) and AMM 49-17 (PP).

The APU insulation panels are not easily damaged, however, it the panels are punctured, you should repair the damaged panel as soon as possible. If you do not repair a puncture immediately, the panel can store fluids in the insulation. If fluids get into the insulation material, you must replace the panel. The air inlet scoop replaces the foam and support insulation (egg crate) that were installed between the APU insulation panels and the bulkhead. During an airplane operation, the air inlet scoop supplies forced outside air through the empty space between the APU insulation panels and the bulkhead for ventilation and for the removal of flammable or unwanted fluids. The location of the air inlet coop is behind the forward insulation panel and adjacent to the forward right side of the APU cowl door.

Source: Boeing training manual

Sorry, my dear... friend, that item it is as simple at it looks-just a simple scoop doing a very basic job: venting. It is very clear in the text posted above. That's it, nothing else  (ah, a last thing, the source of these informations is the B737 Technical Training Manual-if you want one, talk to Boeing).


text:
how does a jet engine work? (explanation for non-aviation related people)
Which day-to-day example would you use to explain a person (who is not involved into the aviation sector) how does a jet engine work? Would objects such as a hairdryer, a vacuum cleaner or a fan be appropriate?

Blow up a balloon and let go. Air rushes out of the back. Newton tells us that every action has an equal and opposite reaction; the balloon goes the other way. How does a jet engine get air to rush out of the back?  It compresses it, injects fuel and burns it.  It burns very quickly and expands.  The only place it can expand it out of the back so it rushes out of there are the jet goes in the opposite direction.  A jet engine is just another internal combustion engine using suck, squeeze, bang, blow

I posted an additional answer specifically for you here. This answer is in very simple words, but accurate. To me this question should not be closed, it targets a different level of explanation than the duplicate.

I vote for leaving this question closed. If the existing answers of a question are not "simple enough", then new answers (or edits) should be added to address a wider audience.

@kevin: Which principle of SE are you referring to?

@mins generally we have a few answers to a question, and sometimes a "TL;DR" section and a detailed section within the same answer. That's my impression anyway - questions are based on topics, not audience levels.

@kevin. In this case, the OP question is mentioning a "day-to-day example" and is asking (maybe more) for an explanation of the reaction principle that is not found at all in the "duplicate".


text:
Coefficient of Parasitic Drag - What is the Preferred Basis of Measurement?
In attempting to determine the coefficient of Parasitic (Zero Lift) Drag - the cD0 - for various aircraft, I have discovered that there appear to be several ways this number is represented: by reference to wing area, frontal area, flat plate area (which may be the same thing) and wetted area.
The standard equation for Parasitic Drag is:
Parasitic Drag = cD0 X dynamic pressure X reference (wing area, etc)
If my understanding is correct, the Parasitic Drag is determined, either experimentally or by adding components together, and the person making the computation can choose the basis of measurement for the cD0.
Is there a preferred basis of measurement?
(It is easiest to use wing area because that is what you use to compute lift and induced drag.)
Is there a way to determine which basis is being used or referenced?
(I have found some equations that use cD0 but do not specify which basis they are using.)
If not, is there a way to "ballpark" the number to guess which one you have?
For example, if I plug a cD0 into a set of equations that compute Parasitic Drag using wing area, I would expect that the cD0 should result in a sea level top speed that approximates the published sea level top speed for the aircraft. If the computed top speed is significantly lower, the cD0 I am using may be for frontal area. If the computed top speed is significantly higher, the cD0 I am using may be for wetted area.
I also read somewhere that, at Vy, the Induced Drag is 3X the Parasitic Drag.  Is that true?

Welcome to Aviation.SE! This platform works best if you only ask one question per thread, so I would advise posting your last question separately.

Sanchises - Thanks. I considered that. In this case, I was asking the question in the context of whether that formula also provided a useful way to "ballpark" the cD0 since Vy is a published number. Also, I was not sure if that would be a stupid question because I have only seen that formula mentioned once.

What is the Preferred Basis of Measurement?

The basis of measurement depends on the method and/or the component whose drag is being evaluated.

As "parasitic drag" I consider here that part of drag which is not proportional to the square of $C_l$.
Parasitic drag can be simply calculated as the sum  of the drag of all the components which make up the aircraft:
$C_{D_{parasitic}}= C_{D_{fuselage}}+C_{D_{wing}}+C_{D_{landing gear}}+C_{D_{nacelle}}+…$
Each term of $C_{D_{parasitic}}$ is normally estimated:

either using known standard historical values from similar design; these values are normally scaled via the basic dimension of the component; for example Raymer¹ gives a $C_d$ of some 0.25 for a wheel and tyre assembly, which has to be multiplied by its frontal area;
or via simplified equations which take as input the basic parameters of the component, like length and diameter for fuselage or thickness and sweep angle for wing; for example again Raymer gives for the fuselage a quite simple equation which depends on its length, diameter, external (wet) surface, Reynolds and Mach numbers.

Some term depends also on $C_l$ (landing gear for example).
Since $C_{D_{parasitic}}$ always refers to wing surface, all of its coefficients have to be divided by $S_{wing}$ before being summed up.

¹ Daniel P. Raymer, Aircraft Design: A Conceptual Approach, AIAA Inc.


text:
Is a compressor blade slowing down the airflow or accelerating it?
When a jet aircraft flies at maximum speed, are compressor blades actually a barrier to the air flow, or is the airflow accelerated by the compressor?

Hello Yayang, welcome to aviation.SE! I tried to rewrite your question in such a way that it is easier understood. Please improve my changes if you want to. I have removed your second question; please only ask one question at a time. You can ask it as a separate question.

I think it's wrong to think of the compressor as a barrier, your taking air and forcing it into a high pressure area.

Note that @DeltaLima dropped the second question. On this site if you want to ask two question, it is strongly preferred to make two posts.

are compressor blades actually a barrier to the air flow, or is the airflow accelerated by the compressor

These are not exhaustive alternatives. Keep in mind that subsonic fluid flow when constricted accelerates and pressure decreases (Bernoulli's principle).
What the compressor does is increase the pressure. The speed decreases slightly.

are compressor blades actually a barrier to the air flow

Actually it's complicated. In a sense they are. The compressor always takes the air at around M0.5 irrespective of flight speed. That means at slower speed the engine needs to suck air from wider stream while at higher speeds some air is spilled over the intake lips. So in a sense, the compressor is a barrier.
But then, the compressor must increase the energy of the stream, otherwise the engine could not produce any thrust. Just the energy goes in pressure, not velocity. So in this sense it is definitely not a barrier.
In fact, the thrust acts mainly on the compressor, because that is the most significant aft-facing surface on which the pressure in the combustion chamber can act.
At supersonic speed it works differently. Now pressure changes can't affect the flow upstream, so the air can't be spilled over the intake lips anymore. Instead now the intake must decelerate it by shock waves and then expansion in a diffusor.
This is needed, because turbomachinery only works well in subsonic flow¹. Since pressure increases as the flow speed decreases, this also increases the overall pressure ratio of the engine and thus its efficiency.
Note that the air also heats up, via adiabatic heating, as it slows down and compresses. Somewhere around M5–6, the air becomes so hot from the compression that adding more heat to it by burning fuel is no longer practical, so above that speed, the combustion must occur in supersonic flow. This is the scramjet engine. It does not have any compressor or turbine, because it gets all the compression from constricting the flow and it turbomachinery wouldn't work at that speed anyway.

Should the compressor be considered a barrier, or is it instead the intake system that slows the initial ram airflow?

@JonathanWalters, at subsonic speed, the compressor, but really the whole engine, is what causes the pressure increase ahead of it. At supersonic speeds pressure changes don't propagate upstream, so there it is the intake shape (supersonic intakes are low while subsonic ones are usually very short) that slows and precompresses the flow.

Jet engine axial type compressors rotor blades do accelerate airflow. It is important to note that the net speed change in velocity is usually negative, that is, airflow through the compressor section taken as a whole is usually slowed.
Additionally, the engine intake generally slows airflow as well while increasing pressure. This occurs within the airflow prior to direct contact with the compressor.
Jet engine axial compressors are comprised of multiple pressure stages with the primary design of increasing pressure, not velocity. Each pressure stage is comprised of pairs of rotors and stators. The rotors consist of rows of compressor blades arranged radially on a rotating spindle. The stators consist of fixed rows of vanes arranged radially and placed after and between the rotors.
The compressor blades of the rotors accelerate the airflow and impart a momentary velocity increase. Subsequently, the stator vanes and the decreasing geometry of the compressor design slow the speed and change the airflow direction (which is also an acceleration). Each pressure stage raises the pressure, typically with a small net decrease in speed.
See the following figure and text excerpt from the Jeppesen A&P Technician Powerplant Handbook:

The task of an axial compressor is to raise air pressure rather than
  air velocity. Therefore, each compressor stage raises the pressure of
  the incoming air while the air's velocity is alternately increased
  then decreased as airflow proceeds through the compressor. The rotor
  blades slightly accelerate the airflow, then the stator vanes diffuse
  the air, slowing it and increasing the pressure. The overall result is
  increased air pressure and relatively constant air velocity from
  compressor inlet to outlet.

Figure 3-29

I would be interested in understanding why this was downvoted. Have I missunderstood the question?

I am not the downvoter. Just a remark, that R/S velocity plot looks strange? Shouldn't be one accelerating and the other decelerating (with corresponding pressure variations) those discontinuities look a bit odd to me. see this answer (but is is about turbines, you have to "revert it") http://aviation.stackexchange.com/questions/20785/how-does-ignited-gas-spin-the-turbine/20790#20790

I was trying to explain the rotor/stator concept to my vehicle mechanic at work to explain why a standard ventilation fan is going to be good at accelerating air but lousy at creating pressure because it lacks stators.. I wasn't doing so well with my explanation and he wasn't buying it. Maybe I can show him your answer.

@Federico Yes, that's true; I hadn't noticed that. The labeling should instead indicate individual pressure stages. Not my work, straight out of the textbook. Perhaps I should fix it.

Following @Federico 's remark, here is a drawing from RR, for the compressor section. Source.

Well some basic math might reveal the answer.
If a turbojet has a compression ratio of say 20:1, gas flow entering the combustion chamber will be at a 20 times higher pressure than the gas flow entering the engine inlet.
If we assume the overall cross sectional area of the gas passage through the stators and rotors in the compressor does not change (this is usually not the case) then the gas flow would have to slow.
Since Mdot = rho * Vdot = rho * A * vdot
and by IGL for simplicity, P = rho * R * T
this gives
Mdot = (P/RT) * A * vdot
Since Mdot and A remain constant and P increases, vdot must decrease to compensate.  While temperature does increase, it is not enough to compensate for the change in pressure, which can be shown with a sensitivity analysis.
Ideally you want to decelerate the gas flow as little as possible as the decrease in momentum creates a drag impulse upon the engine.  Both centrifugal and axial ompressors narrow in volume as gas flows through the core to reduce this.

I don't know of any turbine engines with axial flow compressors with a gas passage that does not decrease in the direction of flow. Usually the decrease is quite substantial.


text:
How much do elevators deflect on takeoff?
Do elevators create immediate lift off on the big airplanes? E.g., on a 737, 747, or an Airbus, I don't see elevators moving up when I see airplanes lifting off in videos. Is it just a slight up movement that is hard to see then?
In other words—for airliners—how big of an elevator deflection is required for takeoff?

What do you mean by "immediate lift off"? You don't need a lot of elevator to make the aircraft pitch up for take-off, and usually once you get the nose up the elevator is pitched back down to level to maintain the climb.

I think perhaps the question is due to the fact that large aircraft have considerable momentum, and there is a delay in applying down elevator and the aircraft rotating to the required angle of attack to begin climbing.

To be more clear I mean what creates the sudden increae of angle of attack as a airplane lifts of the runway which looks like 45 degrees angle-- not immediate lift off. I can only conclude it is the elevators, however, on large airplanes eg 747, I do not see the elevators moving at  on take off in some lift off videos on You Tube.

There is no sudden increase in angle of attack, it's all very gradual. It takes time to rotate to the angle at which take-off occurs, hence why the elevator is not moving at the point of take-off.

Here is another way of saying it --  The airplane speeds up  on the runway -- it is horizontal -- then it suddenly goes into a 45 degree angle of attack lift off. Yet the elevators did not seem to move at all...

Thanks for all the responses. So to keep this simple, it is the elevators in up position that in fact create the 12 degree lift off in the larger airplanes --eg 747, airbus.

Next question is how much do the elevators deflect to create a 12 degree lift off in inches please.


text:
ATSU AOC software for A320 - how to change mass units for OOOI msgs from LBS to KGS
There are 2 A320 acft; one which transmits OOOI msgs with FOB in LBS and one with FOB in KGS.  We want to maintain records in KGS but since the values are auto-parsed into our ops software we have to change them manually each time for the acft that transmits in LBS.  I am not an AME.  I inquired with Honeywell - they said it's a simple change but they'll charge us > $10,000 for it.
Do any Avionics AMEs know how to copy the ATSU AOC software from 1 acft to the other (from the KG acft to the LB acft)?  Is it possible?


text:
What is the cause of Wingtip Stalling? Why do sweptback wings have a tendency of stalling at the tip?
I am a beginner in Aircraft aerodynamics. I have read about the disadvantages of wingtip stalling and the fact that sweptback wings have a greater tendency of stalling at the tips and forward swept wings don't. However, I don't know what causes it to happen. Is it something to do with the direction of airflow from root to tip and vice versa?


text:
Are there any English speaking flight schools to get a PPL in continental Europe?
I'm a UK citizen, planning on getting my PPL this year. My goal was to train intensively, taking lessons several times a week. Unfortunately the UK weather means that even if you have the time and money to train that often, it's easier said than done since lessons are called off more often than not due to a low cloud base or rain.
I was thinking I might be able to get it done faster if I went somewhere like Spain or Italy where the weather is better, but obviously for this to work I'd need to find somewhere with English instruction. I'm assuming if I could find such a place, then as long as the school was EASA certified then the hours would count towards a UK PPL and I could just return home at the end to take my checkride and written exams.
Does my plan make sense, or is there a flaw I've overlooked? If it would work, can anyone recommend a good school that meets these criteria?

I would find a school and call them, chances are they are available in English because the universal ATC language is English.

There certainly are, be carefull that changing schools mid-course can be difficult, and (afairecall) you need to take the ppl exam (in english) with the CAA that approved the ATO. eg, Spanish CAA and Spanish PPL,  which you can then convert to a UK one

Welcome to the site! To prevent the question to be closed as off-topic, be sure to remove "resource location" aspects, and as opinion-based, be sure to not ask for the latter ("Does my plan make sense",  "can anyone recommend a good school").

All PPL's in the UK are now EASA PPLs. Which means any license you get in Europe is valid in the UK (and vice versa).

@Jamiec  I think NPPLs are still being issued and they won't become Part-FCL (LAPL or whatever) until sometime next year

Some years ago there were a couple of flight schools on Malta, I'm not sure if they are still operating, DuckDuckGo could probably tell you
The most common way of getting a JAR (pre-EASA) PPL (for UK trainees) outside the UK used to be to go to a JAR accredited school in the USA. Florida was very popular.  I expect there are schools in the US that are allowed to train to the EASA syllabus these days.  I think you need to travel on a training visa to do this and I'm not sure what is involved in getting one
I think I'm right in saying your flight test can be taken by an EASA examiner anywhere, no need to come home to do it (and anyway you'd want to do it over familiar territory)
A big warning to anyone learning to fly is not to pay upfront.  Pay lesson by lesson.  Flying schools can go bust very quickly, many students have lost a lot of money this way
Personally, I would be wary of training outside the UK, the weather conditions and procedures will be very different from what you are used to, and a low-hours pilot could easily get caught out.  Marginal conditions can go bad on you very quickly.  If you do do this, try to get some supplementary lessons in poorish weather and get some time fly through UK CAS to get used to ATC procedures

I know there's one school in Malaga, Spain that provides instruction and exam in English https://www.oneair.es/  and this place has decent weather year round, so that might be a good option.


text:
Why does lower atmospheric pressure produce higher EPR (thrust)?
Recently, I read manual about engine, PW JT8D-217A/C, and there is a chart making me so confused.
Manual show the reference table tells you what number, EPR, is normal in specific temperature and atmospheric pressure.

(Click to enlarge)
We all know EPR can be related to engine thrust.
I thought higher Atmospheric Pressure meaning higher air mass density, so EPR should be higher.
At lower temperature making higher efficiency (Higher EPR) is quite easy to understand.
But according to the manual, lower Atmospheric Pressure can produce higher thrust when temperature is constant.
Also when temperature is higher than 102.2F, the EPR doesn't change anymore in different atmospheric pressure.
Why is that?

Compression must be reduced at high atmospheric pressure to avoid exceeding the absolute pressure limits for the inside of the engine. A lower pressure ratio results in less thrust.


text:
Can TRACON increase plane speed on approach?
I was listening to Norcal yesterday and heard a controller request an airplane increase speed to 210 knots "if able". The plane was previously at ~180 knots. The pilot responded he had flaps 2 and politely declined.
Can TRACON demand a plane increase speed if the controller already assigned a slower speed?
And if yes, what would be the phraseology? "Maintain 210 knots or greater until DUMBA"? (I don't think I've ever heard that when a plane was slower.)

They can request anything they want, it's up to the pilots to determine if the request can be followed safely or not. Same holds true when requesting a lower speeds that the airplane can't fly.

From what I've heard, a fairly common phraseology for speed control for visual approaches into SFO is "maintain 180 knots to the bridge", but surely that number could be higher if the controller wanted it, and surely the crew could respond "unable" if it posed a problem.

Keep in mind the 200kt speed limit under B airspace in the USA, as TRACONs and even published instrument procedures cannot authorize you to violate it!

Yes, we can ask pilots to speed up on approach, but it is discouraged and not commonly done. This is because, as an aircraft approaches an airport, the crew will configure it for landing. This includes lowering the flaps, which lowers a number of reference speeds - essentially, making them unable to fly very fast. In order to speed up, they would have to raise the flaps again, which would result in extra workload and bad fuel economy.
The controller you heard knows this. They used the phrase "if able" to emphasize to the pilots, that they only had to comply if it was convenient for them.
ICAO provides the following guidance (form Doc 4444):

4.6.1.3    Speed adjustments should be limited to those necessary to establish and/or maintain a desired separation minimum or spacing. Instructions involving frequent changes of speed, including alternate speed increases and decreases, should be avoided.
...
4.6.3.5    Arriving aircraft should be permitted to operate in a clean configuration for as long as possible. Below 4 550 m (FL 150), speed reductions for turbojet aircraft to not less than 410 km/h (220 kt) IAS, which will normally be very close to the minimum speed of turbojet aircraft in a clean configuration, may be used.
4.6.3.6    Only minor speed adjustments not exceeding plus/minus 40 km/h (20 kt) IAS should be used for aircraft on intermediate and final approach.
4.6.3.7    Speed control should not be applied to aircraft after passing a point 7 km (4 NM) from the threshold on final approach.

In my experience, at least, it's pretty common to request increased speed from light aircraft.

@jamesqf from what I've learned (by reading here), that usually happens because said light aircraft is A) landing at a larger and busier airport, B) holding up the works because light aircraft tend to land slowly in comparison to larger aircraft, and C) can generally land at higher speeds by skipping flaps and other low-speed lift devices - essentially "flying" the plane onto the runway as opposed to "landing" on the runway. That said, the light aircraft pilot can still respond "unable" and continue on his merry way.

@FreeMan: Sure, but of course if it's not a large/busy airport (by my standards, anyway), it's not going to have a control tower.  Maybe not even Unicom :-)  But I got pretty good at keeping to cruise speed (which in a Cherokee isn't all that fast :-)) until on short final, then doing a slip and touching down at the right distance to hit the first taxiway.

SOCAL will do this pretty routinely sequencing arrivals into LAX.  The most common practice is to assign 250 knots at some point, then slow you to 210, then assign 170 or 180 "to the marker" (about 5-6 mile final), at which point you'll be slowing to your target approach speed.  So mostly commonly, they are only assigning speed reductions.  However, there are times when they want you to close up space on the aircraft ahead, so they might speed you back up again until the extra distance is closed up -- at which point they slow you back again.
These instructions are always given with the implicit understanding that, if you can't do it, you'll say so... "XYZ123 is unable 250, I can give you 225" or something similar.  If you've already extended flaps so that the higher speed would be near or over an aircraft limitation, that could be a reasonable reply.  (Of course, if you extended flaps 20 miles out from the runway, maybe retracting them again & not annoying SOCAL might be a wiser course of action.)
Depending on the aircraft type (how fast you can fly clean, and how quickly you can slow to your approach speed so that you are stable for the approach at 1000' - or whatever your OpSpecs/SOP's require), you might be given something else.  Light aircraft are often told to keep a fairly high speed (for them) to the marker, since otherwise they're like bicycles on the freeway & slowing down everybody behind them.  On the other hand, they can often decelerate more rapidly than some jets, so staying fast until close in may work out just fine.  Again, if the controller is assigning a speed that's too fast until too close, it's the pilot's job to say, "unable."
The controllers at SOCAL are excellent at knowing what the aircraft they're working with are capable of, and they are very good at timing the speed reductions so that you end up with about 3 miles separation on short final, so that the runway usage is kept high... as one aircraft is turning off the runway, the next jet is a mile or so out.  Much closer than that & you risk go-arounds if somebody gets nervous or the turning-off aircraft isn't expeditious; much farther than that & you aren't landing planes as quickly as what you could be.
Some controllers match the proficiency of the ones at SOCAL; others don't compare.  Of course, you don't really expect the controller at Podunk to have the same practice in setting up a string of aircraft coming in one after another as those in SOCAL, NORCAL, ATL, ORD, and so forth.  The good ones will use all the tools at their disposal, and sometimes this will include having one aircraft increase their speed for a while.


text:
Why do planes overfly Moffett Field before landing?
I live near Moffett Field, and I noticed that some airplanes go over the field once before landing, along the runway (not across), even when the tower is open. I thought that was a procedure for non-towered airports only?


text:
Why doesn't the US have a flag carrier airline?
With the recent United Airlines fiasco, I was reading up on their history and the airline industry and to my surprise I found that United Airlines is not the US flag carrier. In fact, the US does not have flag carrying airline at all! All this time I thought UA was the national carrier for the US. This brings me to the question, why? Why doesn't the US have a national carrier? Is this because of some FAA regulation?

Are you asking why the United States doesn't have a government (approved/sanctioned/official) airline? Or are you asking if the US has a "flag" (US registered) airline? Usually the US government at least tries to stay out of commercial enterprises...

Oh, sorry, I'm asking why the US does not have govt approved/official airline, like the British Airways or Air India

British Airways is not a govt approved/official airline.

Because the US believes in free enterprise (at least in theory - there's always Amtrak :-(), so the government shouldn't be operating businesses.

AFAIK Pan Am and TWA were considered flag carriers until deregulation. They weren't government owned, but they were assigned the international routes that were negotiated by treaty with other countries. Deregulation ended regulatory control over routing and pricing, so the concept of flag carrier in the US disappeared

This is along the same lines of why the U.S. does not have a national car manufacturer, or a national utility. The Airline Deregulation Act (1978) pretty much guaranteed competition, which is difficult to do with a nationally subsidized carrier.

It does, it's called Air United States of America Airways... what, you've never heard of them?  Well they are very small, they don't even have their own football team.

Because that's socialist nonsense! We don't nationalize businesses here. We had airline price regulation, and prices were much more (in 1970 money!), until deregulation in 1978 reduced prices and increased competition. BTW FedEx is not a government business either. (But the USPS is, and when Lysander Spooner tried to compete with them with the American Letter Carrier Company, he was swiftly outgunned and beaten down, so not everything is fair.)

@simon While British Airways might not be "government owned or official", it is still considered to be the "Flag Carrier" for GB.

@Jimmy Only by it's own advertising.  The very definition of flag carrier is that it enjoys preferential rights or subsidies.  BA is by no measure, a flag carrier.  Maggie Thatcher finally cut the last remaining route preferences when they took the Union Flag off the tails.

@Simon Nope. The BA non-Union-Jack tail liveries were introduced in 1997. Margaret Thatcher famously didn't like them but she'd not been Prime Minister or even a Member of Parliament for seven years by then.

The definition of a "Flag Carrier", although somewhat vague and frequently disputed, is generally accepted to mean: an air carrier that is owned by, or subsidized by, the government of the country of registration, especially for the purposes of flying international routes.
Wikipedia:

A flag carrier is a transportation company, such as an airline or
  shipping company, that, being locally registered in a given state,
  enjoys preferential rights or privileges accorded by the government
  for international operations. The term also refers to any carrier
  that is or was owned by a government, even long after their
  privatization when preferential rights or privileges continue.

Business Dictionary:

Airline ... registered under the laws of a country whose government gives it partial or total monopoly over international routes.

The US has no carriers in this category. All subsidizing of major domestic and international routes ended with the Airline Deregulation Act of 1978. (Note: there are still some small airlines and routes that are subsidized to serve very small communities... If you want to read more about this program, it's called Essential Air Service.)

I really hope our "Flag Carriers" are not flown by the Dept. of State

Darn that pesky free enterprise thing.

So, I mean what was the logical reasoning for arriving at the Airline Deregulation act? Did something event force them for this? Where can I read more about the events that led to this act?

Wow. over 50 years of various political events, 6 or 7 changes to air transport laws, 4 different government bodies being created, changed, and abolished, various accidents, and changing societal values. This entire "US History of Aviation" eventually led to the deregulation act. There is no "any one thing", nor can it be "easily" explained within the context of this website. One good book: Contrived Competition: Regulation and Deregulation in America. (can read parts of it on google books).  Talks about deregulating several industries in the 70s, with the airlines being just one of those.

Oh great! Thank you @Jimmy

The definitions you quote directly contradict your claim. You say that the definition is "owned by, or subsidized by, the government" but one of your quotes doesn't even mention ownership and the other one only mentions it as the second alternative.

There are many meanings of "flag carrier", one of which is not addressed in the other answers.
In regulated international air travel markets, each pair of countries would agree (via treaty) to let each others' "flag" carriers fly between them.  For instance, Britain would allow Air France (France's flag carrier) to fly to London in return for France allowing British Airways (Britain's flag carrier) to fly to Paris.  Where a country owned and/or subsidized one particular airline, that was the obvious choice as the flag carrier, but that is the only connection.
The US, for various reasons, was allowed to name two flag carriers: Pan-Am and TWA.  Later deregulation of the most profitable markets left them vulnerable to destructive competition, and they eventually folded.  United inherited Pan-Am's flag status, and American inherited TWA's flag status.  That doesn't matter in the many deregulated markets, of course, but it still matters for the many markets that are still regulated.  Non-flag carriers, such as Delta, can only access regulated markets via a codeshare agreement with the other country's flag carrier.

All US carriers operating on a type 401 certificate are referred to as flag carriers. No subsidy issues are part of consideration

We were recently told these were the Flag Carriers of the US, under the Fly America First:

Federal travelers are required by 49 U.S.C. 40118, commonly referred to as the "Fly America Act," to use U.S. air carrier service for all air travel and cargo transportation services funded by the U.S. government.

My company gave us this list:

U.S. flag air carriers:
  • Airtran Airways (FL)      • Alaska Airlines (AS) • American Airlines (AA)
  • Continental Airlines (CO) • Delta Airlines (DL)  • Frontier Airlines (F9)
  • Hawaiian Airlines (HA)    • JetBlue Airways (B6) • Midwest Express (YX)
  • Southwest Airlines (WN)   • Spirit Airlines (NK) • United Airlines (UA)
  • USAirways (US)

With plenty of exceptions relating to international travel.
From vanderbilt.edu/ocga/docs/vupolicies/FLYAMERICAACT.pdf

U.S. Flag Air Carrier. The term "U.S. flag air carrier" means an air carrier holding a certificate under section 401 of the Federal Aviation Act of 1958 (49 U.S.C. App. 1371). Foreign air carriers operating under permits are excluded.

I quoted from this:  https://www.gsa.gov/policy-regulations/policy/travel-management-policy/fly-america-act  and was given the list of carriers by my company.

"U.S. Flag Air Carrier. The term "U.S. flag air carrier" means an air carrier holding a certificate under section 401 of the Federal Aviation Act of 1958 (49 U.S.C. App. 1371). Foreign air carriers operating under permits are excluded."  from https://www.vanderbilt.edu/ocga/docs/vupolicies/FLYAMERICAACT.pdf

I think that perhaps someone had done the research to determine which airlines were certified under section 401. Our company is on the small side and uses a travel agent to book flights, with no deals that I know of as we have never been to told to use one airline over another.  More like, 'if you want to be reimbursed for your flight with no issues, you shall use one of these carriers'.

Those are not "flag carriers" in the sense meant by the question.  They are simply American companies.  You might use the same logic to claim that Ford and GM are "flag" automakers.

Ford & GM are public transportation companies tho, so not really an apt comparison. Maybe a bus company, or a rail company. Uber? Lyft?

No, those are what one might call "carriers who fly under the American flag". That's not the same as a "flag carrier".

@CrossRoads: That was my point.  Ford and GM are US-based companies that make cars, those listed airlines are likewise US-based companies that fly airplanes.  What the OP has is nothing more than an instance of "Buy American", exactly the same as if Federal agencies were required to buy Ford & Chevy vehicles rather than Hondas and Toyotas.


text:
Why is Boeing being blamed for the recent 737 Max crashes?
Boeing has received more than 5,000 orders for the aircraft, it has delivered 350 so far and only 2 of them have crashed. That's not a very high crash rate. The FAA, until just a day ago, was very confident that the aircraft is "fly worthy".
The Ethiopian crash investigators haven't even ascertained if the crash was due to a problem with the aircraft or if it was pilot error and already the whole world is assuming it was Boeing and has grounded flights worldwide. Why is this?
Why is Boeing being blamed for this? Flights crash all the time! Sure, not as much today as they did in the beginning, but they still do! We can't go on grounding every aircraft that crashed during flight! Plus the aircraft was in development for 5 years - I'm sure buyers were allowed to test the aircraft to their satisfaction before placing orders, Boeing themselves would've rigorously tested the aircraft, it was even approved by the FAA!
Shouldn't we make sure it was not the pilot's fault before we blame the manufacturer? Or did they already do that? I tried to find articles that explained WHY everyone is rallying against the company, but couldn't find anything concrete.

No crashes do not happen all the time. 2 crash of the same model of jet in such a short time is literally unheard off.

A precautionary grounding is NOT the same as appointing blame.

Possibly relevant question: What does the preliminary report on the Ethiopian B737 MAX accident show?

The Ethiopian crash was in a new plane, in good weather, with multiple witnesses describing rattling noises and a smoke trail.
We can rule out weather, and pilot error seems unlikely to cause that.
The plane is so new that maintenance is not likely to be a factor either.
Blaming Boeing may not be fully justified at this point, but caution over the literally hundreds of thousands of passengers who fly on them every day is not inappropriate either.


text:
How do I use the GNS 620 Trainer provided by Garmin?
I downloaded the Trainer for the Garmin GNS 620 here.
I installed and now have a few programs. I can open the GNS 620, the touchscreen GPS and boot it up, plan a route and so on.
There's also a program to open the G600, the PFD and MFD. However, when I open this program, it just says it can't connect to a simulator and exits.
My questions is: How can I use those trainers properly? How can I simulate the flight? How do I control the virtual plane? Do I need a simulator for this?
I would really appreciate answers, as I have no idea what to do.
Thanks!

Welcome to aviation.SE! Is there a manual installed with the software? Even if someone here knows the trainer well, explaining how to use a complex piece of software isn't really a good fit for a Q&A site like this.

I found all the information needed in the "Trainer's User Guide.pdf" that was installed with the software.
In short, launch ONLY the GTN - G600 Trainer.  Do not have any other components started.
3 windows should launch: GTN Trainer, G600 Trainer and Navigation Control.
Give it a good 30 seconds to load up, then on the GTN Trainer window, click "Power On".
All windows should come up and be active.


text:
Why is powering on the APU so low on the A320 emergency checklist?
In the movie Sully we have this section of dialogue

He wasn't following proper procedure at all.
  And I know because I had the QRH in my hands.
  He switched on the APU immediately after
  engine blowback.
  According to the Airbus that's the 15th thing on the list to do. 15th!
  If he had followed the damn rules, we'd all be dead.

Now, another question discusses the importance of the APU to the crash, but my question is, why is it so far down the list? I suspect the above is dramatized somewhat, but it does seem odd that it's so low. What does Airbus consider more important in their handbook?

Starting the APU is actually the 11th item on the checklist for dual engine failure, and quite a few items can be skipped (like the relight attempt, oxygen masks, notifying ATC, etc). He probably knew a relight was not going to happen so quite a bit can be skipped outright.

Here are the excerpts from the QRH for the A320, dual engine failure.

@MichaelKjörling Sully seemed very accurate on the terminology aspect. It wasn't Star Trek, where the names are made up, and the points don't matter.

@ymb1 It would probably be fairly easy to make separate checklists for, say, dual engine failure below 10,000 ft (where supplemental oxygen is unnecessary even for extended periods) and dual engine failure above 10,000 ft (where supplemental oxygen starts to become needed, depending on duration of operations, but you're able to glide farther). I guess I personally hope that Airbus will respond by making such checklists, plus one specifically for dual engine failure during early climbout; possibly by renaming the existing one "dual engine failure above 10,000 ft" or some such.

Seems like I remember hearing somewhere that, in response to US1549, Airbus moved the APU start up in the checklist

The assumption was that a multiple engine failure happens at altitude, for instance in cruise while flying through volcanic ashes. This happened to British Airways Flight 9 and to KLM flight 876, with all four engines switching off due to the ash cloud. Total engine failure at altitude buys time, and if time is available first priority is to attempt to re-light an engine. APU will probably be off during cruise, and given the choice to re-light an engine or to light the APU it is clearly the engine that is the most beneficial to start.
All-engine-failure in airliners is very very unlikely. It is a function of number of engines - in a four-engined aeroplane, the chance of all four engines failing is assumed to be super extremely incredibly unlikely. With a statistical failure rate of 10$^{-5}$ per flying hour per engine, a four-engined aeroplane would experience all engine failure due to statistical causes once in $\frac{10^{20}}{24}$ flying hours1: a billion times a billion times four. So very uncommon that the only practical consideration would be a common mode failure from external causes, such as the volcanic ash cloud.
For a twin-engine like the A320, chance of all engine failure due to random causes is 2 * 10$^{-10}$, still considered an Extremely Unlikely event. Procedures are written for the least unlikely events, and updated according to events that actually happened. Now that a dual engine failure during take-off has actually happened, updating the procedures will be the result of failure analyses: how likely would it be to happen again, or what other procedures can be put in place to avoid this situation.
 1: The chance that any of four engines fails is 4 * 10$^{-5}$ per flying hour. Then 3 * 10$^{-5}$ per flying hour for the remaining engines etc.

In the volcanic-ash scenario, you describe in your answer, would immediately starting the APU cause the APU to ingest volcanic ash, too?

Yes it would, and it would cause loss of valuable time in getting the engines restarted.

I think the 10^20 is not correct because the multiplication of the faillure rates assumes that all engine failures are independent statistical events. This does not account for an event that affects all engines at the same time (like the big flock of birds in case of the Hudson!)

@ROIMaison indeed, a common mode failure, like stated in the answer.


text:
Aside from the Stratolaunch, how many other aircraft have split bodies?
So there's a lot of excitement about the new (and massive) Stratolaunch

Split bodies are uncommon (although launching spacecraft from them commercially is a new thing). The only other airframe that I've seen like this would be Virgin's White Knight 2 (also a spacecraft launcher). Do any other aircraft use a split body like this?

Related

Depends on how you define it but there have been a variety over the years:

The P-38 Lighting has 3 bodies there was also a later experimental version the XP-49 that only saw a single airframe.
The F-82 was a a twin fuselage version of the P-51
The boomerang which was also designed by Burt Rutan (the designer of the stratolaunch)
A one off twin fuselage piper cub was built but never saw production scale.
As @DanPichelman points out in the comments, the Yak-110

...although launching spacecraft from them commercially is a new thing

This is not entirely true, piggy backing rocket planes on larger aircraft has been widely done since the 50's

The Yak-110 also comes to mind.

Don't forget the He 111Z